aw: Null check glBindRenderbufferEXT binding

Some devices are apparently crashing due to a null binding here.
Plumbing the correct conditions is somewhat difficult and brittle. So
just add a null check instead.

Bug: 950472
Change-Id: I26b9d1c2ec0db7abe23a32f2e8d7078a59e5767a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1564570Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649854}

android_webview/browser/gfx/scoped_app_gl_state_restore.cc

@@ -199,8 +199,11 @@ ScopedAppGLStateRestoreImpl::ScopedAppGLStateRestoreImpl(
     SaveHWUIState();
   }
 
-  if (mode_ == ScopedAppGLStateRestore::MODE_RESOURCE_MANAGEMENT) {
+  if (mode_ == ScopedAppGLStateRestore::MODE_RESOURCE_MANAGEMENT &&
+      ::gl::g_current_gl_driver->fn.glBindRenderbufferEXTFn != nullptr) {
     // Android 5.0.0 specific qualcomm workaround. See crbug.com/434570.
+    // Null check the binding since checking the proper condition is hard. See
+    // crbug.com/950472.
     glBindRenderbufferEXT(GL_RENDERBUFFER, 0);
   }